It is well known in prior art of the superior light transmission properties of ZBLAN fiberoptic waveguides (a.k.a. fiber) as well as its application in fiber lasers and amplifiers. Unfortunately, all ZBLAN fiber-forming methods involve fabrication from a melt, which creates inherent problems such as the formation of bubbles, core-clad interface irregularities and inclusions. The ZBLAN fiber drawing process generally occurs at 310° C. in a controlled atmosphere (to minimize contamination by moisture or oxygen impurities which significantly weaken the fiber) using a narrow heat zone compared to silica glass. Drawing is complicated by a small difference (only 124° C.) between the glass transition temperature (approximately 260° C.) and the melting temperature (approximately 310° C.). As a result, ZBLAN fibers often contain undesired crystallites. It is known that the crystallite concentration can be reduced or eliminated by melting and resolidifying ZBLAN in zero gravity (a.k.a. microgravity). The theory is that microgravity conditions reduce convection processes that cause crystallite formation in ZBLAN glasses.
The disclosed subject matter helps to avoid this and other problems.
Known art, such as French patent application Nos. 76.18878 and 77.09618, discloses fabricating a ZBLAN optical fiber in 1 G (normal gravity). However, such known methods of fabricating ZBLAN optical fibers often contain undesired crystallites. These optical fibers may suffer from reduced light transmission and, in the case of use in fiber lasers, undesirable heat generation and an associated upper power limit.
Known art, such as U.S. Pat. No. 2,749,255 by Nack, et. al., discloses cladding a glass fiber with a higher melting temperature cladding via a cladding system comprised of a fiber metalizing system employing nickel carbonyl or gas plating methods. The advantage of this gas plating method is that the metallic cladding (e.g. nickel plating) deposition occurs at a lower temperature (approximately 180-250° C.) than, for example, the ZBLAN glass transition temperature (approximately 260° C.) and the melting point of the ZBLAN glass material (approximately 310° C.)
Additional known art, such as U.S. Pat. No. 5,991,486 by Braglia, discloses an optical fiber that has the core made of a rare earth doped non-oxide glass and cladding made of an oxide glass. The glass of the core has a melting temperature lower than that of the glass of the cladding and lying within the range of the softening temperatures of the cladding. To produce the fiber, a preform, obtained by introducing an element made of the non-oxide glass into the hole of a capillary tube made of the oxide glass, is brought to a temperature lying within the range of softening temperatures of the oxide glass and not lower than the melting temperature of the non-oxide glass, and is drawn. The capillary tube, during the drawing process, serves as a container for the molten glass of the core.